Why is Association of South-East Asian Nations (ASEAN) a strategic geographical location for investment?

Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, Thailand and Vietnam together form one of the world's largest emerging markets. Annual growth in that region is 5 percent and expecting to grow at that rate till 2020. However, investors still tend to look at ASEAN as at 10 separated countries. Multinational corporations based outside of Southeastern region use ASEAN as a test market to reach Chinese or Indian customers or as a base of low-cost labor.